Spectroscopic study of plasma during electrolytic oxidation of magnesium-aluminium alloys

Plasma during Electrolytic Oxidation (PEO) of magnesium-aluminium alloys is studied in this work by means of Optical Emission Spectroscopy (OES). Spectral line shapes of the H_β, Al II 704.21 nm and Mg II 448.11 nm line are analyzed to measure plasma electron number density N_e. From the H_β line profile, two PEO processes characterized by relatively low electron number densities N_e ≈ 10¹⁵ cm⁻³ and N_e ≈ 2 × 10¹⁶ cm⁻³ were discovered while the shape and shift of Al II and Mg II lines revealed the third process characterized by large electron density N_e = (1-2) × 10¹⁷ cm⁻³. Low N_e processes, related with breakdown in gas bubbles and on oxide surface, are not influenced by anode material or electrolyte composition. The ejection of evaporated anode material through oxide layer is designated here as third PEO process. Using the Boltzmann plot technique, electron temperature of 4000 K and 33000 K is determined from relative intensities of Mg I and O II lines, respectively. Several difficulties in the analysis of spectral line shapes are met during this study and the ways to overcome some of the obstacles are demonstrated